The following abbreviations are utilized throughout this document:    3GPP Third Generation Partnership Project    AGW Access Gateway    AS Application Server    BSSAP Base Station System Application Part    BSSMAP Base Station System Management Application Part    CM Connection Management    CSoLTE Circuit Switched Services over LTE Radio Access    CSoLTE-D CS over LTE Decoupled    CSoLTE-I CS over LTE Integrated    CSoLTEvGAN CS over LTE utilizing GAN Protocols    CS Circuit-Switched    DTM Dual Transfer Mode    E-CGI E-UTRAN Cell Global Identifier    eMSC-S evolved MSC Server    EPC Evolved Packet Core    EPS Evolved Packet System    E-UTRAN Evolved UTRAN    FDMA Frequency Division Multiple Access    GA-CSR Generic Access, Circuit Switched Resources    GAN Generic Access Network    GANC Generic Access Network Controller    GAN-CGI GAN Cell Global Identifier    GA-RC Generic Access Resource Control    GSM Global System for Mobile Communications    IASA Inter-Access Anchor    IMS IP Multimedia Subsystem    IWU Interworking Unit    LAI Location Area Identifier    LTE Long Term Evolution    LU Location Update    MME Mobility Management Entity    MO Mobile Originated    MT Mobile Terminated    MSC-S Mobile Switching Center Server    MSS Mobile Softswitch Solution    NAS Non Access Stratum    OFDM Orthogonal Frequency Division Multiplexing    PCRF Policy Charging Rule Function    PDN Packet Data Network    PMSC Packet MSC    PCSC Packet CS Controller    P-GW Packet Data Network Gateway    PS Packet-Switched    RRC Radio Resource Control    SAE System Architecture Evolution    SAE-GW SAE Gateway    SAI Service Area Identifier    SCCP Signaling Connection Control Part    SC-FDMA Single Carrier Frequency Division Multiple Access    SEGW Security Gateway    SGSN Serving GPRS Support Node    S-GW Serving Gateway    SMS Short Message Service    TA Tracking Area    TAU Tracking Area Update    UE User Equipment    UMA Unlicensed Mobile Access    UPE User Plane Entity    UTRAN Universal Terrestrial Radio Access Network    VLR Visitor Location Register    WCDMA Wideband Code Division Multiple Access
Mobile CS services based on GSM and WCDMA radio access are a world-wide success story and provide telecommunication services with a single subscription in almost all countries of the world. The number of CS subscribers is still growing rapidly, boosted by the rollout of mobile CS services in dense population countries such as India and China. This success story is furthermore extended by the evolution of the classical MSC architecture into a softswitch solution, which utilizes a packet transport infrastructure for mobile CS services.
Recently, the 3GPP work item “Evolved UTRA and UTRAN” (i.e., E-UTRAN, started in summer 2006) defined a Long-Term Evolution (LTE) concept that assures competitiveness of 3GPP-based access technology. It was preceded by an extensive evaluation phase of possible features and techniques in the RAN workgroups that concluded that the agreed system concepts can meet most of the requirements and no significant issue was identified in terms of feasibility.
LTE utilizes OFDM radio technology in the downlink and SC-FDMA for the uplink, allowing at least 100 Mbps peak data rate for downlink data rate and 50 Mbps for uplink data rate. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world.
In parallel with the LTE RAN (E-UTRAN) standardization, 3GPP also drives a System Architecture Evolution (SAE) work item to develop an Evolved Packet Core (EPC) network. The E-UTRAN and EPC together build up the Evolved Packet System (EPS). The SAE core network is made up of core nodes, which may be grouped into Control Plane (Mobility Management Entity or MME) nodes and User Plane nodes such as Serving Gateway (S-GW) and Packet Data Network Gateway (PDN GW or P-GW). In this document, a co-location of the S-GW and the P-GW is denoted Access GW (AGW).
FIG. 1 is a simplified block diagram of nodes in a conventional LTE/SAE network architecture 10. The SAE CN includes core nodes, which may be further split into a Control Plane Mobility Management Entity (MME) node 11 and a User Plane SAE Gateway (SAE-GW) node 12. In the terminology currently used, the SAE-GW contains both User Plane Entity (UPE) and Inter-Access Anchor (IASA) functionality. The SAE-GW also has two different roles defined: Serving Gateway 13 and Packet Data Network (PDN) Gateway 14. The term SAE-GW is used herein for both the Serving GW and the PDN GW. The MME 11 is connected to the E-UTRAN 15 via an S1-MME interface 16, and the SAE-GW 12 is connected to the E-UTRAN via an S1-U interface 17. The SAE architecture is further described in 3GPP TS 23.401 and 23.402.
Common to both LTE and SAE is that only a Packet Switched (PS) domain was initially to be specified, i.e., all services were to be supported via the PS domain. GSM (via DTM) and WCDMA, however, provide both PS and CS access simultaneously. Thus, if telephony services are to be deployed over LTE radio access, an IMS-based service engine is mandatory. It has been recently investigated how to use LTE/SAE as access technology to the existing Mobile Softswitch Solution (MSS) infrastructure. This work, referred to as “CS over LTE” (CSoLTE) or the longer name “CS domain services over evolved PS access,” is documented in 3GPP TR 23.879 and in 3GPP TS 23.272.
FIG. 2 is a simplified block diagram of a CSoLTE general architecture 20. A Packet MSC (PMSC) 21 serves both traditional 2G and 3G RANs 22 and the CSoLTE solutions through the LTE E-UTRAN 15. The PMSC contains two new logical functions: a Packet CS Controller (PCSC) 23 and an Interworking Unit (IWU) 24. In addition, there is an SGs interface 25 between the MME 11 and an MSC Server (MSC-S) 26. This interface is used for Paging and Mobility Management (MM) signaling to attach a mobile station (MS) 27 in the MSC-S based on, for example, SAE MM procedures performed between the terminal and the MME using procedures similar to those for the Gs-interface between the MSC and SGSN in existing GSM and WCDMA networks and defined in 3GPP TS 29.016 and 29.018. The protocol used in the Gs-interface is called BSSAP+ and uses connectionless SCCP and normal MTP layers (or M3UA with SIGTRAN) in the existing implementations.
The communication between the MS 27 and the PMSC 21 is based on the SGi interface. This means that all direct communication between the MS and the PCSC 23 and the IWU 24 in the PMSC is based on IP protocols, and that the MS is visible and reachable using an IP-address via the SAE-GW 12 (FIG. 1). This communication is divided into two different interfaces: U8c for the control plane and U8u for the user plane. The PCSC has also an Rx interface to a Policy Charging Rule Function (PCRF) 28 for allocation of LTE/SAE bearers.
FIG. 3 is a simplified block diagram of the CSoLTE architecture illustrating the interfaces in more detail.
With reference to FIGS. 1-3, three different embodiments for providing CSoLTE service are described below. The first embodiment is called “CS Fallback” and means that the MS 27 is performing SAE MM procedures towards the MME 13 while camping on LTE access. For example, the MME registers the MS in the MSC-S 26 for CS-based services using the SGs interface 25 shown in FIG. 2. When a page for CS services is received in the MSC-S, the page is forwarded via the SGs interface to the MME 11 and then to the MS, which performs fallback to the 2G or 3G RANs 22. The fallback can be based on PS HO, Cell Change order, or terminal-based selection of the suitable cell in the 2G or 3G RAN. Similar behavior applies for Mobile Originated (MO) CS services. When these are triggered and the MS is camping on LTE access, the MS falls back to the 2G or 3G RANs and triggers the initiation of the CS service there.
The second embodiment is called CS over LTE Integrated (CSoLTE-I). In this embodiment, the same SAE MM procedures as for “CS Fallback” are used over the SGs interface 25, but instead of performing fallback to the 2G or 3G RANs, the MS performs all the CS services over the LTE E-UTRAN 15. This means that the CS services (also called Connection Management (CM) procedures) are transported over IP-based protocols between the PMSC 21 and the MS over the U8c and U8u interfaces using the LTE E-UTRAN and the SAE nodes such as the SAE-GW 12.
The third embodiment is called CS over LTE Decoupled (CSoLTE-D). In this embodiment, both MM and CM procedures are transported over IP-based protocols directly between the PMSC 21 and the terminal 27 over the U8c and U8u interfaces using the LTE E-UTRAN 15 and the SAE user plane nodes such as the SAE-GW 12.
FIG. 4 illustrates the control plane protocol architecture (i.e., the U8c interface) between the MS 27 and the PMSC 21.
FIG. 5 illustrates the user plane protocol architecture (i.e. the U8u interface) between the MS 27 and the PMSC 21.
FIG. 6 is a functional block diagram of an existing Generic Access Network (GAN) as defined in 3GPP TS 43.318 and TS 44.318. 3GPP has standardized the Generic Access Network (GAN)-concept starting from 3GPP Release-6. The more correct name is “Generic Access to A/Gb Interfaces” and this standardization was based on the Unlicensed Mobile Access (UMA) de-facto specifications.
The GAN provides a new Radio Access Network (RAN) and the node corresponding to the GERAN BSC is called a Generic Access Network Controller (GANC). The basic principle is that the MS is a dual-mode, dual-radio handset including for example both WiFi and 3GPP-macro radio support (GSM, WCDMA, or both). The MS connects to a WiFi Access Point (AP) (not shown) using the WiFi Radio. The GAN standard defines for example how the MS can function in GAN mode and access the services provided by the GSM Core Network (CN) using the Up-interface between the MS and the GANC.
The initial GAN standard can be called “2G-GAN” or “GSM-GAN” since the standard GSM interfaces, A and Gb, are used between the GANC and the CN 37. In addition, work is ongoing to standardize a “3G-GAN” or “WCDMA-GAN” solution. In this case, the GANC will use the standard WCDMA interfaces, for example the lu-cs and the lu-ps interfaces, to connect to the CN. The resulting standard can be also called “Generic Access to A/Gb Interfaces” or shortly “GAN-lu”.
FIG. 7 illustrates the CS Domain Control Plane Protocol Architecture related to GAN and the Up-interface 36. The GANC uses the normal A-interface signaling towards the MSC 38 and interworks the related protocol, such as BSSAP, towards the relevant GAN-protocols, such as GA-CSR (Generic Access, Circuit Switched Resources), in both directions.
FIG. 8 is a functional block diagram of the CSoLTEvGAN architecture. The CSoLTEvGAN solution has not yet been standardized, but exists in 3GPP TR 23.879 as one of the alternatives for CS service support over LTE. TR 23.879 covers a number of different alternatives. This alternative basically views LTE as a Generic Access Network and utilizes the GAN protocols for the control and user planes.